In circulating fluidized bed boilers, fly ash, limestone and other particulate materials become entrained in the flue gas. These hot materials are later separated out from the flue gas and are collected in a storage hopper for subsequent recycling via a standpipe back to the fluidizing chamber or furnace of the boiler.
When the temperature of these particulate materials changes rapidly, or when the flow rate is resumed after a period long enough to allow them to cool down, extremely high temperature transients occur in the conduit from the standpipe to the furnace. Also, during normal operation, this mix of particulate material flows somewhat horizontally in the conduit from the standpipe to the furnace. Consequently, the bulk of the flow occurs along the top of the conduit while the solids along the bottom are essentially stagnant, moving very slowly and intermittently or not at all. This phenomenon is inherent in that two substances with greatly different densities are being conveyed; solids, such as sand or limestone, and gas, such as air or flue gas. The effect of such stagnation at the lower part of the horizontal conduit occurs more significantly at lower flow rates and can occur at all flow rates depending on specific conditions. This stagnation and the corresponding extreme temperature differentials which occur results in a deterioration of the conduit itself.
In many boilers, it is common to use a non-mechanical valve or a controlled particulate discharge valve intermediate the particle storage hopper and the furnace to control the flow of the particulate materials. Such a valve uses a small amount of measured air to control these particles. Furthermore, for proper operation this type of valve also requires a length of horizontal conduit long enough to at least stop the flow of the particulate materials when aeration control is shut off. However, this length cannot be too long since an excessive length only increases flow resistance thereby reducing the maximum volume of flow attainable. Generally, an ideal length is one slightly longer than the normal angle of repose of the particulate materials being conveyed.
It is thus an object of this invention to provide an improved means for conveying recycled particulate materials within a circulating fluidized bed boiler. Another object of this invention is to keep the recycled particles at a uniform temperature within the horizontal conduit thereby avoiding the problems associated with extreme temperature differentials. A further object of this invention is to free the relationship of horizontal transport distance and maximum solids flow capacity. Still another object of this invention is to provide a means for discharging a known quantity of particles into the furnace. Yet another object of this invention is to create a recycling rate which is self-compensating i.e., the greater the flow of particulate materials, the greater the discharge of these particulates into the furnace. These and other objects will become apparent upon further investigation of this invention.